T cells developing in the thymus (pre-T cells) are destined to become T α/β cells through rearrangement of the TCR β gene initially, followed by the TCR α gene (Figure 2–5). If unproductive rearrangements of TCR genes occur (nonfunctional TCR α or β proteins), apoptosis of these pre-T cells follows (Figure 2–5A). If functional rearrangements of TCR α and β proteins occur, cells express TCR α/β dimer and CD3 molecules at low levels on the cell surface. TCR-rearranged cells proliferate 100-fold. Positive and negative selection occurs based on the ability of the rearranged TCR α/β to recognize antigenic peptides in association with self-MHC molecules on thymic epithelial and dendritic cells. Negative selection (clonal deletion) appears to take place in the thymus medulla, where pre-T cells–bearing TCRs specific for self peptides bound to self-MHC molecules are deleted. At least 97% of developing T cells undergo apoptosis within the thymus (central tolerance). Positively selected pre-T cells increase expression of TCR α/β, express either CD4 or CD8, and become mature T cells. These mature T cells exit the thymus and go to the periphery. CD4 T cells are activated in the periphery in an MHC class II-restricted fashion, while CD8 T cells are activated in an MHC class I-restricted fashion.
A. Central T-cell tolerance: Mechanisms of central tolerance (at the thymus level) are depicted. From top to bottom, pre-T cells first rearrange their TCR. Unproductive (nonfunctional) rearrangements lead to apoptosis, while productive ones engage pre-T cells in self-antigen recognition. Clonal deletion indicates elimination of cells based on their high or no avidity for self antigen (apoptosis). Surviving low-avidity cells reach the periphery as mature CD4 and CD8 cells. B. Peripheral T-cell tolerance: May be accomplished through any of the five depicted mechanisms. 1. Clonal deletion: After encountering self antigen in the context of self-MHC molecules and simultaneous delivery of a second signal (CD80/86–CD28) by APCs (top left), autoreactive T cells become activated. These activated T cells express Fas molecules on their surface but are resistant to Fas ligand (FasL)-mediated apoptosis because of the simultaneous expression of Bcl-xL (not shown) induced by CD28 ligation during activation. Several days after activation, when Bcl-xL presence has declined, CD4 cells become susceptible to FasL-mediated apoptosis. Natural killer cells (NK-T) may then accomplish the task of eliminating these autoreactive T cells. 2. Anergy: Anergy may be induced via CD80/86–CD152 interaction 48 to 72 hours following activation or may result from the lack of a second costimulatory signal from APCs presenting self antigen (nonprofessional APCs). 3. Active suppression: Active suppression is thought to occur when nonhematopoietic cells (stimulated by IFN-γ) present antigen in an MHC class II–restricted fashion to CD4 T-suppressor cells (TS, also known as CD4 + CD25 + FOXP3 + regulatory T cells, T regs). Before becoming unresponsive, these cells may induce specific CD8 TS cells. In turn, these CD8 TS cells may suppress antigen-specific autoreactive T cells. 4. Ignorance (top right): Some autoreactive T cells may never encounter self antigen because it may be sequestered from the immune system. Although they may persist in the circulation, they never become activated. 5. Immune deviation: Under specific circumstances, noninflammatory TH2 responses could suppress inflammatory (autoreactive) TH1 responses (see text).
A differential avidity model in which the fate of T cells is determined by the intrinsic affinity of TCRs for their ligands has been advanced to explain the paradox between positive and negative selection. According to this model, T cells with high avidity for MHC-self peptide complexes would be eliminated (negative selection), whereas T cells with low avidity to MHC-self peptide complexes would be positively selected. If the avidity is close to zero, T cells would not be selected (for lack of effective signal to survive). The biochemical factor or factors that signal survival (low avidity of TCR binding) versus apoptosis (triggered by high avidity interactions) have yet to be found.
Costimulatory interactions between CD28 and CD80/86 and between CD154, CD40, and adhesion molecules, such as lymphocyte function-associated antigen-1 (LFA-1), are also involved in preferential deletion of self-reactive T cells in the medullary region of the thymus. It is known that negative selection is not 100% effective and that some potentially autoreactive T cells do escape to the periphery. Not all self peptides would be presented to pre-T cells during their development in the thymus. Self peptides derived from secluded proteins (ie, intracytoplasmic enzymes) only timely expressed after rigid regulatory control (ie, puberty) in endocrine glands are believed to be a likely source. Therefore, the peripheral immune system must maintain tolerance through complementary control mechanisms.
“Peripheral tolerance” (Figure 2–5B) may be maintained by the induction of unresponsiveness to self antigen (anergy) or by the induction of regulatory T cells (T regs), such as suppressor T cells (active suppression). Peripheral clonal deletion (apoptosis) of autoreactive T cells that have escaped from the thymus may play an important role in limiting rapidly expanding responses, but there are many examples where autoreactive T cells persist. Some autoreactive T cells may never encounter the self antigen because it may be sequestered from the immune system (ignorance). Lastly, immune deviation, whereby noninflammatory TH2 responses suppress an autoreactive inflammatory TH1 response, inducing peripheral tolerance, deserves further discussion. TH1 cells, which regulate cell-mediated responses, secrete IFN-γ and small amounts of IL-4. In contrast, TH2 cells, which provide help for antibody production, secrete abundant IL-4 and little IFN-γ. A prevailing concept in human autoimmunity is that TH1 responses are believed to dominate. It has been shown in animal models that induction of TH2 responses ameliorates TH1 responses. Hence, unbalanced TH1 immune deviation may lead to a breakage of peripheral tolerance. However, evidence to the contrary exists in some endocrinopathies. (See autoimmune response in the section on Autoimmune Aspects of Thyroid Disease, later in the chapter.)
Clonal deletion and anergy occur through apoptosis at the site of activation or after passage through the liver. High antigen dose and chronic stimulation induce peripheral elimination of both CD4 and CD8 T cells. Activated T cells express Fas molecules on their surfaces but are resistant to FasL-mediated apoptosis because of the simultaneous expression of Bcl-xL (apoptosis-resistance molecules), induced by CD28 ligation during activation (see Immune Recognition and Response, earlier in the chapter). Several days after activation, when Bcl-xL has declined, CD4 cells become susceptible to Fas-mediated apoptosis (activation-induced cell death; AICD). A similar mechanism via p75 tumor necrosis factor (TNF) receptor has been shown for CD8 cells. Therefore, autoreactive T cells might be deleted by apoptosis induced by chronic stimulation with self antigens, present abundantly in the periphery. However, autoreactive T cells specific for very rare self antigens may be difficult to eliminate.
Anergy also results from the lack of a second costimulatory signal. When nonhematopoietic cells stimulated by IFN-γ present antigen in an MHC class II-restricted fashion (as thyrocytes do in AITD), autoreactive T cells may be rendered unresponsive because of the absence of a CD28–CD80/86-mediated signal (nonhematopoietic cells do not express CD80/86 as professional APCs do). However, even if the two signals are provided, anergy may result from the lack of TH cell-originated cytokines (IL-2, -4, -7, etc). It has also been shown that in vivo T-cell anergy may be induced by CD80/86–CD152 interaction (see also Immune Recognition and Response, discussed earlier).
T-cell active suppression is considered to be a major regulatory mechanism of peripheral tolerance; however, its mode of action is still under study. As mentioned above, nonhematopoietic cells stimulated by IFN-γ present antigen in an MHC class II-restricted fashion to T cells and render them anergic. These nonhematopoietic cells (nonprofessional APCs) may also present to CD4 T-suppressor cells (TS, also known as CD4 + CD25 + FOXP3 + regulatory T cells, T regs). Before becoming unresponsive, these cells may induce specific CD8 T suppressor (TS) cells. In turn, these CD8 TS cells may regulate (via T-cell-suppressor factors or cytotoxicity) autoreactive T cells (see also Figure 2–5B).
Instead of the thymus, the bone marrow provides the setting for central B-cell tolerance. Pre-B cells rearrange their B-cell receptor (BCR or membrane-bound immunoglobulin) early in development. The immunoglobulin heavy (H) chain genes rearrange first, followed by light (L) chain gene rearrangement. Unproductive rearrangements and pairings leading to formation of nonfunctional immunoglobulin drive pre-B cells to apoptosis (Figure 2–6A). Functional rearrangements (functional BCRs) allow immature B-cell expansion and expression of IgM and CD21 (a marker of functionality). Only one-third of the precursor cells reach this stage. The random rearrangement of the V, D, and J segments of immunoglobulin genes during this period inevitably generates self-recognizing immunoglobulins. Negative selection of autoreactive B cells occurs at the immature B cell stage on the basis of the avidity of the BCR for self antigens. Similar to the T-cell clonal deletion, immature B cells that strongly bind antigens in the bone marrow are eliminated by apoptosis. Some autoreactive immature B cells, instead of undergoing apoptosis, resume rearrangements of their L-chain genes in an attempt to reassemble new κ or λ genes. This procedure, called BCR editing, permanently inactivates the autoreactive immunoglobulin genes. Soluble antigens, presumably because they generate weaker signals through the BCR of immature B cells, do not cause apoptosis but render cells unresponsive to stimuli (anergy). These anergic B cells migrate to the periphery, where they express IgD. They may be activated under special circumstances, making anergy less than sufficient as a mechanism of enforcing tolerance. Only immature B cells in the bone marrow with no avidity for antigens (membrane-bound or soluble) become mature B cells with the capacity to express both IgM and IgD. As with T cells, 97% of developing B cells undergo apoptosis within the bone marrow. Also, and as with T cells, central clonal deletion, anergy, and BCR editing eliminates autoreactive B cells, recognizing bone marrow-derived self antigens.
A. Central B-cell tolerance: As T cells do in the thymus, B-cells rearrange their B-cell receptor (BCR) in the bone marrow. Unproductive rearrangements drive pre-B cells to apoptosis. Functional rearrangements allow expansion and expression of IgM. Next, similar to T-cell clonal deletion, immature B cells that strongly bind self antigens in the bone marrow are eliminated by apoptosis. Some autoreactive immature B cells, instead of becoming apoptotic, however, resume rearrangements of their L-chain genes, attempting to reassemble new allelic κ or λ genes (BCR editing). Soluble self antigens presumably generate weaker signals through the BCR of immature B cells; they do not cause apoptosis but make cells unresponsive to stimuli (anergy). These anergic B cells migrate to the periphery, expressing IgD, and may be activated under special circumstances. Only immature B cells with no avidity for antigens become mature B cells, expressing both IgM and IgD. These are the predominant cells that make it to the periphery. B. Peripheral B-cell tolerance: In the “absence” of antigen (top right), mature B cells are actively eliminated by activated T cells via Fas–FasL and CD40–CD154 interactions. In the “presence” of specific self antigen but “without T-cell help,” antigen recognition by BCRs induces apoptosis or anergy on mature B cells. If self antigen and specific autoreactive T-cell help are provided, two events develop (center): (1) The B cell becomes an IgM-secreting plasma cell (top left), and, in the presence of the appropriate cytokines after expression of CD40 (for TH cell CD154 interaction), class switching occurs (bottom left). (2) Further somatic hypermutation of the Ig-variable region genes, which changes affinity of BCRs, occurs. Mutants with low-affinity receptors undergo apoptosis, while improved-affinity BCRs are positively selected. In the presence of CD40 ligation of CD154, antigen-stimulated B cells become memory B cells. These two events are the same as in foreign antigen recognition.
Peripheral B-cell tolerance (Figure 2–6B) is also crucial for protection against autoimmunity. It appears that in the absence of antigen, mature B cells are actively eliminated in the periphery by activated T cells via Fas–FasL and CD40–CD154 interactions. In the presence of specific antigen but without T-cell help, antigen recognition by BCRs induces apoptosis or anergy of mature B cells. If antigen and specific T-cell help are provided—that is, if antigen bound to the BCR is internalized, processed, and presented in an MHC class II-restricted fashion to a previously activated TH cell specific for the same antigen—two events occur. One, the B cell becomes an IgM-secreting plasma cell, and—in the presence of the appropriate cytokines and after expression of CD40 (for TH cell CD154 interaction)—class switching occurs. Two, further somatic hypermutation of the immunoglobulin variable region genes of such mature B cells, which changes affinity of BCRs for antigens, also occurs in germinal centers (see also Immune Recognition and Response, discussed earlier). Mutants with low-affinity receptors undergo apoptosis, while enhanced-affinity BCRs are positively selected. In the presence of CD40 ligation of CD154, antigen-stimulated B cells become memory B cells (see Figure 2–6B).
The ability of mature B cells to capture very low quantities of antigen via high-affinity BCRs allows them to amplify their antigen-presenting capacity to more than 1000 times that of other professional APCs. This particular property may become critical in the development of chronic organ-specific autoimmune diseases in which the source of antigen is limited. Thus, autoreactive B cells that happen to escape the control mechanisms described could amplify and perpetuate autoimmune responses in patients with failing endocrine organs when tissue destruction has left only minute amounts of residual antigen.